1. Field of the Invention
The present invention relates generally to a device for measuring the aerodynamic size and electrical charge of airborne particles and in particular, to a device that subjects aerosols to an acoustic field and to an electric field and thereafter senses the motion of individual airborne particles through the use of a laser Doppler velocimeter.
2. Description of the Prior Art
Measurement of the charge and size of aerosol particles has been sought using a variety of different devices. For instance, an article entitled "An Aerosol Analyzer" by Joseph I. Masters, 24 Review of Scientific Instruments 586 (August 1953), describes a device for collecting charged aerosol particles on filter paper and thereafter measuring the average charge of the particles collected. The charge distribution of the particles cannot be determined, however, nor can the presence of bi-polar charges.
An article entitled "Electrical Mobility of Sub-micron Particles" by W. J. Megaw et al, 219 Nature 259 (July 20, 1968), describes a device for accumulating deposits in a copper foil lined box through which a radioactive aerosol is passed so that the electrical mobility of the sample can be determined. Filtered air is passed in around the aerosol inlet so that it is possible to adjust the thickness of the flow stream.
Lewis et al U.S. Pat. No. 4,375,673 discloses a charge spectrograph wherein a uniform electric field is applied to charged toner particles by a series of rectangular electrodes and the particles collected on filter paper so that the average size and charge can be found from the deflections of the particles.
Mazumder et al, "Simultaneous Measurements of Aerodynamic Diameter and Electrostatic Charge on a Single Particle Basis" in Measurement of Suspended Particles by Quasi-Elastic Light Scattering 327 (B. Dahneke ed. 1983) discloses a method for analyzing aerosol particles wherein an oscillating electric field is applied to the particle and its size and charge are determined from the phase lag of the particle as compared to the electric field. The device uses a laser Doppler velocimeter to detect the particles' motion. However, uncharged particles are not detectable by this device.
Additional prior art includes: Millikan, R. A. (1909). 79th Meeting of the Br. Assoc. Adv. Sci., Winnepeg, Manitoba; T. Gillespie, et. al., "An Instrument for Determining the Electric Charge Distribution in Aerosols," 30 Canadian Jour. of Chemistry 1056 (Dec. 1952); U.S. Pat. Nos. 2,537,628, Hanson et. al.; 3,208,286, Richard; 3,723,712, Komline, Sr. et. al.; and 3,944,797, Coulter et. al.